The invention relates to a double row angular contact ball bearing, the special structure of which is particularly well suited to the mounting of gears to rotate with respect to supports, particularly gearboxes and transmission gearboxes and especially helicopter auxiliary or main transmission gearboxes.
The invention also relates to a cantilever mounting of gears on a special-purpose bearing according to the invention, particularly for mounting helicopter main or auxiliary transmission gearbox gears, and the invention finally relates to a modular assembly comprising at least one gear, a specialized purpose bearing according to the invention, and a device for preloading the bearing, on which the gear is cantilever mounted, according to the special-purpose mounting according to the present invention.
In helicopter transmission gearboxes, whether these be main transmission gearboxes inserted between the propulsion units, generally turbo engine units, and the main rotors, and arranged as at least one step-down stage, for reducing the rotational speed of the output shaft or shafts of the propulsion units to a nominal speed at which the main rotors are rotated, or whether these be auxiliary transmission gearboxes inserted between rear transmission shafts connected to take-offs or auxiliary outputs from main transmission gearboxes, and tail or anti-torque rotors for driving the latter in rotation, the gears of such transmission gearboxes are generally supported by at least two rolling bearings, the rolling bodies of which may be balls or rollers, generally cylindrical or conical ones.
One of the bearings may be mounted at the xe2x80x9cnosexe2x80x9d of the gear (on the opposite side to the side via which the gear is driven in rotation), while the other bearing is mounted at the xe2x80x9ctailxe2x80x9d of the gear (on the side via which the gear is driven in rotation), but the gear may also be mounted on the same side of at least two rolling bearings which, irrespective of the mounting, reacts the radial loads while just one of the bearings reacts the axial load.
When the mounting of a gear entails precise axial positioning, as may be the case with a spiral bevel gear, the meshing conditions of which may entail either a minimum clearance or no axial clearance, particularly when the level of dynamic loadings on the gear is high, bearings are combined in what are known as xe2x80x9cOxe2x80x9d or xe2x80x9cXxe2x80x9d configurations, preloaded by adjustment with angular contact ball bearings or taper roller bearings.
Whether the rolling bodies in the bearings are tapered rollers or balls, it is known that xe2x80x9cOxe2x80x9d bearings are combinations of at least two bearings such that, if the normals to the points of contact between the rolling bodies and the raceways of the corresponding races are considered, the intersections of these normals with the axis of the bearings delimit an axial segment of a length that exceeds the axial distance joining the extreme points of contact (axially farthest apart) of the rolling bodies, so that the said normals to the points of contact delimit, about the axis of the associated bearings, a pseudo xe2x80x9cOxe2x80x9d.
In the case of an xe2x80x9cXxe2x80x9d configuration of bearings, the said normals to the points of contact between the rolling bodies and the races of the bearings diverge and are directed radially and axially towards the outside of the bearings, so as to delimit, on the common axis of the bearings, a segment of a length shorter than the axial distance joining the extreme points of contact (in reverse set-up by comparison with an xe2x80x9cOxe2x80x9d configuration of bearings).
The preloading of two associated bearings, for example two angular contact ball bearings mounted in opposition (xe2x80x9cOxe2x80x9d configuration), is achieved after shims or spacers of variable and adjustable axial thickness have been arranged between the inner races and the outer races of the two bearings, so that, in the presence of such suitable shims or spacers, the axial clamping of the inner race of a bearing against the corresponding shims and towards the inner race of the other bearing, bearing against a shoulder, imposes a xe2x80x9cno loadxe2x80x9d pressure on the two bearings. Under these conditions of mounting of this arrangement, the application of external forces to the bearings leads to a reduction in the axial preload, which means that the bearings can run with a small or practically zero clearance, corresponding to the required conditions. This preload entails that the axial dimensions of the spacers or shims fitted, for example peel shims, are determined after a succession of mountings and measurements. This results in lengthy and tricky adjustment of the preload, this drawback adding to the one ensuing from the need to fit the said spacers or shims in order to control the preload.
In order to dispense with the need to adjust the preload, it is known practice for the two bearings, which are matched and the internal geometry of which is defined to obtain the desired preload by construction, to be mounted back to back and for the shims or spacers and adjustments thus to be eliminated. However, the two bearings are then likely to be too heavily loaded, and it is known practice for this drawback to be remedied by fitting a third bearing, generally a cylindrical roller bearing, on the line of shafts, to provide additional support and thus avoid cantilever mounting.
To react the axial loads exerted on gears while at the same time mounting them for rotation, it has also already been envisaged for use to be made of other types of bearing, particularly ball bearing variants of the xe2x80x9cdeep groovexe2x80x9d type with three or four points of contact (of the balls with the races), of which one of the two races, generally the inner race, is split into two half-races.
In a deep groove bearing it is known that the raceway of each race, inner or outer, has just one curvature, which is not necessarily the same for both races. Because of this or these curvatures, one consequence of the radial clearance, needed for such a bearing to run correctly, is that such a bearing has fairly large axial clearances which do not allow sufficiently precise mounting, particularly of gears in the axial direction.
To avoid these disadvantages, it has been proposed for use to be made of bearings of the xe2x80x9cthree-point contactxe2x80x9d type, in which one of the races, generally the inner race, is made in two parts so that the centres of curvature of the two raceway grooves formed respectively in the two parts of the split race are offset by an axial distance which corresponds to a saving in axial clearance, or alternatively xe2x80x9cfour-point contactxe2x80x9d bearings, similar to xe2x80x9cthree-point contactxe2x80x9d bearings as regards the race which is split into two parts, whereas in the other race, which is of one piece, the raceway groove is machined with an ogee-shaped cross section having two radii of curvature with centres which are axially offset, each with respect to the other and both with respect to the centre of the balls.
This results in bearings which are more precise and have a smaller axial clearance than conventional deep groove bearings.
However, among the various solutions known in the prior art for reacting the axial loads exerted on gears, the first solution, which consists in using ball bearings of the three-point or four-point contact type, has the drawback of leaving degrees of freedom in the axial direction.
A second solution, which consists in using a preloaded mounting of two associated bearings, which are tapered roller bearings or angular contact ball bearings, to cancel these degrees of freedom in the axial direction, does, on the other hand, have the drawback of entailing adjustment of the preload by insertion of shims of variable and adjustable axial thickness. Furthermore, in the case of mounting xe2x80x9cOxe2x80x9d configured or xe2x80x9cXxe2x80x9d configured bearings, it is necessary to adjust the preload while at the same time making sure that the gear is positioned precisely in the axial direction, this making mounting operations more complicated and increasing the costs of assembly in production and of maintenance.
Note that the use of two bearings, one of which is mounted at the xe2x80x9cnosexe2x80x9d of the gear and the other of which is mounted at the xe2x80x9ctailxe2x80x9d of the gear, may have the drawback of prohibitive bulk, because mounting a bearing at the xe2x80x9cnosexe2x80x9d of the gear and its size are not always compatible with a sufficiently small diameter, at the end of the gear, to allow a grinding wheel in to grind the teeth, in the case of the gear being a spiral bevel gear.
Finally, the last solution, which consists in using a mounting of two bearings back to back and preloaded, supplemented by a third bearing providing an additional support, has the disadvantages that the mass and bulk of the assembly are high, and that it is trickier to mount and entails a longer amount of maintenance time, which is a disadvantage in terms of maintenance.
The problem underlying the invention is to provide a bearing which is particularly suited to gearbox and transmission gearbox architectures observing small bulk and in which the mountings and removals of the gears are quick and easy, the mounting of the gears allowing a significant saving in axial bulk and a saving in weight, and an increase in the precision with which the sets of teeth of the gears can be axially positioned.
To this end, the invention provides a double row angular contact ball bearing, of a known type, comprising:
a one-piece outer race, having two internal raceways arranged substantially in opposition and formed of raceway grooves the concave side of which faces radially towards the axis of the bearing and each of which faces axially towards one respective side of the bearing,
an inner race, consisting of two matched inner half-races, arranged side by side, able to be axially preloaded against each other when the bearing is mounted, and each having an external raceway formed of a raceway groove the concave side of which faces radially towards the outside of the bearing and in such a way that the external raceway of each inner half-race substantially faces a respective one of the two internal raceways of the outer race,
two rows of balls, in which the balls in each row are arranged between the two raceways of a respective one of the two pairs of opposing raceways, and in angular contact with the said raceways, and
at least one ball cage holding the balls of a row of balls and arranged between the outer race and the corresponding inner half-race, wherein the balls of one of the two rows are of a diameter greater than the diameter of the balls of the other row, so that the bearing has two asymmetric rows of balls.
To make it easier to preload, without adjusting, the bearing and to mount it on a shaft of a gear, the two inner half-races advantageously have an internal bore of the same diameter.
Advantageously also, the two inner half-races may have external raceways of substantially the same diameter measured at the bottom of the corresponding raceway groove, and/or the raceway groove forming the external raceway of each of the two inner half-races may have a concave side also facing axially towards the other inner half-race.
To limit its mass and its bulk, the one-piece outer race may advantageously have two axial end parts with a cylindrical or cylindro-conical external face, of which one, surrounding the row of larger-diameter balls, has an outside diameter greater than the outside diameter of the other part, surrounding the smaller-diameter row of balls.
To make the bearing easier to mount on a support, such as the housing of a transmission gearbox, the one-piece outer race may advantageously have a fastening flange projecting radially outwards. When the one-piece outer race has two axial end parts with a cylindrical or cylindro-conical external face and different diameters, as set out hereinabove, the fastening flange advantageously projects radially substantially between the said two axial end parts.
In such a bearing, it is advantageous for at least one ball cage and preferably each of them when the bearing has two ball cages, to be centred about at least one cylindrical bearing surface on the external face of the corresponding inner half-race.
Another subject of the invention is a mounting of at least one gear in a rolling bearing, intended for mounting the gear to rotate with respect to a support, such as a helicopter main or auxiliary transmission gearbox housing, and the principle of the mounting according to the invention relies on the use of a single double row angular contact ball bearing in which the two rows are asymmetric, preloaded without adjustment, according to the invention and as defined hereinabove, and in which the gear is cantilever mounted so that, by comparison with the embodiments of the prior art, in which the gear is mounted between two bearings, one of which is at the xe2x80x9cnosexe2x80x9d of the gear and the other of which is at the xe2x80x9ctailxe2x80x9d of the gear, the bearing according to the invention is mounted, also according to the invention, at the xe2x80x9ctailxe2x80x9d of the gear, the xe2x80x9cnosexe2x80x9d bearing being omitted.
More specifically, the mounting of at least one gear in a rolling bearing is characterized, according to the invention, in that a set of teeth of the gear is secured coaxially to one axial end of a shaft pushed coaxially into the two inner half-races of a double row angular contact ball bearing in which the two rows of balls are asymmetric, according to the invention, the gear cantilevering out from the bearing mounted in such a way that the row of larger-diameter balls is arranged on the same side as the set of teeth of the gear, and that the outer race of the bearing is able to be fixed to the said support.
In running, the row of balls closest to the set of teeth of the gear is the most heavily loaded, which is why the balls in this row are of a diameter larger than the diameter of the balls of the other row, which essentially reacts the preload forces. Each of the two asymmetric rows of balls is thus dimensioned according to the loads applied, which makes it possible to reduce the axial bulk and the mass of the bearing.
When the gear has the overall form of a bevel gear, the bearing is advantageously arranged axially offset with respect to the set of teeth, on the larger-diameter end of the bevel gear teeth.
This mounting no longer exhibits the drawbacks of the mountings known from the prior art and set out hereinabove. In particular, the fact that the single bearing used consists of a double row of angular contact balls arranged in opposition, and which can be preloaded, allows axial and radial loads to be reacted while at the same time allowing the gear to be mounted with precise axial clearance, or even with no axial clearance. The one-piece outer race can easily be fixed to the housing, particularly when the said race has the aforementioned flange. The two half-races of the inner race are matched by the bearing manufacturer to obtain the desired preload once the bearing has been mounted on the shaft of the gear, and the internal geometry of the bearing is, in the known way, defined to take account of the mounting and running conditions, particularly the type of mounting of the bearing on the shaft, by interference or shrink-fitting, and the conditions of expansion, temperature, etc.
Advantageously, the mounting of the gear according to the invention is such that before fixing the outer race of the bearing to the support, the two inner half-races are axially preloaded against each other, when the bearing is mounted on the shaft of the gear, using a preloading device, preferably of the type comprising at least one nut and a member for locking the said nut, such as a lock nut, at least the said nut being screwed onto a threaded part of the shaft, it being possible for the said threaded part to project axially with respect to the bearing on the opposite side to the set of teeth of the gear.
The mounting of a gear on a bearing according to the invention therefore entails no adjustment, the preload being provided by design.
The cantilever mounting of the gear according to the invention, with the type of bearing specific to the invention, is of benefit mainly in helicopter transmission gearboxes, for which this mounting of a gear and of the bearing is particularly suited to transmission gearbox architectures which entail small sizes and ease of mounting and removal. In this application, it is advantageous for the shaft of the gear to be tubular and comprise driving means, such as axial splines, for driving the gear in rotation about the axis of its shaft by means of a driving member, such as a drive shaft, fitted with complementary axial splines.
When the shaft of the gear is a shaft common to two gears, and is therefore secured to two sets of teeth, arranged one on each side of the bearing, the bearing is mounted in such a way that the row of larger-diameter balls faces axially towards that one of the sets of teeth which, in service, is axially loaded the more heavily.
A final subject of the invention is a modular assembly, characterized in that it comprises:
at least one gear comprising a set of teeth secured coaxially to an axial end of a shaft,
a bearing according to the invention and as defined hereinabove, mounted on the shaft of the gear in such a way that the shaft is pushed into the two inner half-races of the bearing and that the row of larger-diameter balls is arranged towards the set of teeth of the gear, and
a preloading device equipping the shaft of the gear and/or the bearing for axially preloading the two inner half-races of the bearing.
The use of such a modular assembly considerably simplifies the mounting and removal of a gear by comparison with the embodiments of the prior art, and a gear thus equipped and associated with such a double row angular contact ball bearing with asymmetric rows of balls allows a significant saving to be made in terms of axial bulk and in terms of mass. Furthermore, the precision on the axial position of the set of teeth of the gear is increased by means of the reduction in the accumulation of dimensions by comparison with the aforementioned mountings of the prior art.